1. Field of the Invention
The present invention generally relates to a heating device, and more particularly, to a high-frequency heating device for driving a magnetron, using the high frequency between 20 and about 50 KHz.
2. Description of the Prior Art
Recently, various kinds of high-frequency heating devices are proposed for the tendency towards miniaturization, lightweight, cost reduction or the like of a power transformer provided in each device.
A typical circuit diagram of the high-frequency heating device is generally shown in FIG. 1a. In the circuit of FIG. 1a, electric power produced by a commercial power source 1 is rectified by a diode bridge 2 to form a unilateral power source. An inductor 3 and a capacitor 4 play in cooperation with each other a role as a filter with respect to the high-frequency switching operation of an inverter, which is comprised of a resonance capacitor 5, a step-up transformer 6, a transistor 7, a diode 8 and a driving circuit 9. The transformer 6 is provided with three windings, the first, second and third windings 10, 11 and 12.
Base current supplied from the driving circuit 9 causes the transistor 7 to perform the switching operation in a predetermined period and duty (the ratio of "on" period with respect to the cycle period). As a result, collector current Ic of the transistor 7 and current of the diode 8 flow as shown in FIG. 1b. On the other hand, when the transistor 7 is off, resonance between the capacitor 5 and the first winding 10 of the transformer 6 generates voltage Vce between a collector and an emitter of the transistor 7, as shown in FIG. 1c. Because of this, high-frequency electric power is generated in the first winding 10.
In the transformer 6, high-frequency high-voltage power and high-frequency low-voltage power are generated in the second winding 11 and the third winding 12, respectively. More specifically, the high-frequency high-voltage power is rectified by a capacitor 37 and a diode 38 to be supplied between an anode and a cathode of a magnetron 15 whereas the high-frequency low-voltage power is supplied to a cathode heater of the magnetron 15. Accordingly, the magnetron 15 oscillates to effect dielectric heating. The magnetron 15 is comprised of the magnetron body 15a and a filter for preventing undesirable radiation towards outside space through heater electrodes of the magnetron body 15a. The filter is comprised of feed-through capacitors 16, 17, 18 and choke coils 19, 20, with the feed-through capacitors 16, 17 and 18 being integrally formed with one another.
In the high-frequency heating device of the above described construction, the sectional area of a core of the step-up transformer 6 can be reduced with the increase of frequency of the power applied between opposite ends of the first winding 10. Therefore, for example, when the inverter is operated with the use of the frequency between 20 and approximately 50 KHz, the weight and size of the step-up transformer 6 can be reduced to a half or less, or to a tenth or less as compared with the case where the step-up is done using the frequency of the commercial power source as it is. This is advantageous in that the power source portion can be manufactured at reduced cost.
The high-frequency heating device employing therein audio frequency below 20 KHz is not serviceable due to undesirable sound generated primarily by the step-up transformer 6.
FIGS. 2a and 2b depict the structure of the aforementioned filter provided in the magnetron 15.
As shown in FIGS. 2a and 2b, the choke coils 19, 20 are accommodated in a filter box 36 of the magnetron 15, and the integrally formed feed-through capacitors 16, 17 and 18 extend through the filter box 36. In the conventional magnetron, electric wires forming the choke coils 19 and 20 have a diameter between 1.4 and 1.6 mm whereas rod-shaped magnetic cores 32 and 33 are made of ferrite having a diameter of 5 mm and a length of 21 mm. The electric wires 30 and 31 are densely wound substantially around the central portion of respective rod-shaped magnetic cores 32 and 33. Each of the electric wires 30 and 31 of respective choke coils 19 and 20 is connected at its one end to one heater electrode 34 of the magnetron body 15a and at its other end to one electrode 35 of the feed-through capacitors 16, 17 and 18.
In the case where the magnetron 15 having the structure of the choke coils 19 and 20 and driven normally with the use of the commercial frequency is driven using the frequency between 20 and 50 KHz much higher than the commercial frequency, the electric wires 30 and 31 of the choke coils 19 and 20 and the magnetic cores 32 and 33 generate heat due to the increase of copper loss caused by skin effect and the increase of core loss, for example, eddy current loss, hysteresis loss or the like.
When the magnetic cores 32 and 33 of ferrite exceed the Curie point in temperature, the permeability of the ferrite rapidly decreases, thus resulting in that the choke coils 19 and 20 can not function as a filter due to the reduction of their inductances. Furthermore, the choke coils 19 and 20 can hardly restrain the electric current flowing in the heater of the magnetron 15 which has been so far restrained thereby, also due to the reduction of their inductances. Accordingly, an excessive current flows through the heater of the magnetron 15 and remarkably shortens its life. In addition, heat generated in the choke coils 19 and 20 raises the temperature of the feed-through capacitors 16, 17 and 18 through the electrodes 35 of these capacitors 16, 17 and 18. Generally speaking, the life of the feed-through capacitor is nearly halved with the temperature rise of approximately 10.degree. C. The kind of insulation of the electric wires 30 and 31 used in the choke coils 19 and 20 is a grade H having the maximum allowable temperature of 180.degree. C. The choke coils 19 and 20 are, therefore, required to be used below this maximum allowable temperature.